The present invention relates to a method for controlling the air-fuel ratio in a carburetor of an internal combustion engine.
Traditionally, an air-fuel ratio in a carburetor is controlled by a needle valve driven by an actuator and adapted for changing the amount of air bled into the carburetor. When the amount of air bled into the carburetor is cut off by fully closing the needle valve, the fuel in a fuel passage in the carburetor tends to stick to the inner wall of an air bleed passage or flow into the air bleed passage because of columnar vibration and pulsating flow created in the fuel passage of the carburetor. Under these circumstances, when a control signal enters the actuator to initiate the control operation of the amount of bleed air, the fuel sticking to the inside wall of the air bleed passage or flowing into the air bleed passage is supplied to an engine together with the bleed air, with the result that the ratio of fuel to air (which is hereinafter referred to as the air-fuel ratio) of the supplied fuel mixture is temporarily enriched and the control responsiveness of the air-fuel ratio is reduced, so that the control characteristics of the air-fuel ratio becomes unstable. Moreover, because the needle valve tends to axially vibrate in fully closing operation thereof, the air-fuel ratio may not be maintained at a constant level in fully closing operation and thereby the control characteristics of the air-fuel ratio becomes disadvantageously unstable.